1. Field of the Invention
The invention described herein relates to bandwidth management, and relates in particular to the provision of bandwidth allocation information to user devices.
2. Background Art
Communications over a cable infrastructure can be governed by a version of the Data Over Cable Service Interface Specification (DOCSIS). In a DOCSIS-based system, or in any two-way grant-based system, communication from an end user device (such as a cable modem, or CM) to a central controller (such as a cable modem termination system, or CMTS) is known as upstream communication. Generally, if a cable modem wishes to transmit upstream, it first makes a request to the central controller, stating the amount of data it wants to send. Because several CMs may be seeking upstream bandwidth, the request may be made in upstream contention opportunities. Requests can also be made as piggybacked requests that are included in the header of an upstream data transmission.
In scheduling upstream transmissions, a CMTS needs to consider the various upstream requests from CMs along with the necessary scheduling of unsolicited grants for applications like voice over internet protocol (VoIP), synchronization, ranging opportunities, and request opportunities. For each upstream channel, the CMTS sends a grant message in the downstream (called a MAP message in DOCSIS). This message defines the various upstream transmission opportunities on that upstream channel over an interval of time. The opportunities may consist of grants to individual CMs, ranging opportunities to individual CMs, request opportunities to particular CMs, broadcast ranging opportunities, broadcast request opportunities to multiple CMs, or any mix of these. The MAP message is the “map” through which the CMTS conveys how the upstream bandwidth is to be used for a particular upstream channel over a defined interval of time. This message states when, for a given upstream channel, each CM is permitted to transmit. CMs must receive the MAP message applicable to the upstream channel(s) that they are utilizing. Each CM then processes the MAP message to find specific transmission opportunities (time slot(s) or a combination of timeslots and spreading codes) that are applicable to or usable by the CM, and to prepare for upstream transmission. A CM therefore needs to have completed processing a MAP prior to the time at which the CM is to transmit upstream, this time having been granted and specified in the MAP.
DOCSIS versions 1.0, 1.1, and 2.0 currently specify a parameter known as MAP processing time. In order for the CMTS to determine how early to send a MAP message in advance of the time allocations stated therein, several delay components are considered. One of the delay components is the time required by the CM to process the MAP and to prepare for a burst transmission upstream. This time delay is generally referred to herein as the grant message time (or MAP processing time, in the DOCSIS context). DOCSIS 1.0 bound the MAP processing time to 200 microseconds. In DOCSIS 2.0, additional time was added to the 200 microseconds for cases in which upstream byte interleaving was enabled for time division multiple access (TDMA), or when synchronous code division multiple access (S-CDMA) mode was used. The DOCSIS 2.0 specifies exactly how to calculate this additional latency, which is added to the 200 microseconds. The sum is still referred to as MAP processing time.
For further advances to DOCSIS, such as that being worked on for the DOCSIS 3.0 project, it is expected that additional time will have to be specified for MAP processing time. One of the reasons is that in DOCSIS 3.0, a CM may use multiple upstream channels in a scheme known as upstream channel bonding. Thus, a CM may be processing multiple MAP messages for different upstream channels at the same time. In addition, a CM may have processing that must be done ahead of time, prior to transmission upstream. In DOCSIS 3.0, for example, continuous concatenation and fragmentation (CCF) may be implemented, requiring such advance processing.
Because different CMs may operate under different DOCSIS versions, CMs may therefore have different processing times for a given MAP message. In addition, different CMs may have different manufacturers, different architectures, and different processors, further affecting MAP processing times.
The conventional solution to this is to send the MAP message sufficiently early, so that all CMs will have completed MAP processing prior to the first grant. This assures that all CMs will be ready to transmit in their allotted time allocations. In fact, this approach assures that all CMs will have completed MAP processing prior to any of the granted timeslots.
While this approach solves the problem of making sure that each CM will be ready to transmit before its allotted time, this approach is also inefficient. The roundtrip between the transmission of a MAP message from a CMTS to a CM, and the transmission upstream by the CM in the allotted bandwidth, can take considerable time for a given CM. A CM may have to wait in an idle state for an extended period between completion of MAP processing and the time at which it can transmit upstream.
What is needed, therefore, is a system and method by which this roundtrip can be reduced, thereby providing more efficient use of time and bandwidth in the allocation of upstream grants to devices such as cable modems.